1. Field of the Invention
The present invention relates to a frame interpolation apparatus and a frame interpolation method for interpolating an image between two frames of a video signal.
2. Description of the Related Art
A disadvantage of a hold-type display device such as a liquid crystal display (LCD), in which the same image is held on display for one frame period, is that although the human eye tracks the movement of a moving object in the image continuously, the object itself moves discontinuously, one frame at a time, causing the edges of the moving object appear blurred. One possible solution to this problem is to perform frame interpolation to increase the frame rate, thereby producing a smooth visual perception of moving objects.
Film-to-video conversion of movies and the like causes another problem. Because of the different frame rates of film and video, a single image frame taken from film is sometimes displayed in two consecutive video fields and sometimes in three consecutive video fields, resulting in a type of jerky motion known as judder.
A similar problem occurs when a computer-processed moving image is converted to a video signal, because two video fields are created from the same computer-processed frame.
Conventional frame interpolation methods include zero-order hold (ZOH) interpolation, in which the image from the preceding frame is used as the interpolated image, and mean value interpolation, in which each picture element (pixel) in the interpolated image is obtained by averaging the same pixels in the preceding and following frames. Zero-order hold interpolation cannot solve the blurring problem of a hold-type display, because it does not provide a smooth visual perception of an object moving in a fixed direction. Mean value interpolation has the problem of producing double images or ‘ghosts’ of moving objects.
A more sophisticated interpolation method detects motion by dividing the frame preceding the interpolated frame, for example, into blocks of picture elements (pixels) finding, for each of these pixel blocks, the most highly correlated pixel block in any position in the frame following the interpolated frame, and creating a pixel block in the interpolated frame from the correlated pair of pixel blocks (see, for example, Japanese Patent Application Publication No. 2004-357215, p. 9, FIG. 16). In this method, however, although the interpolated frame is built up a pixel block at a time, the interpolated pixel blocks usually do not fit together in uniform pattern. Generating the necessary read and write addresses in the image data memory (SDRAM is used in general) and ensuing that interpolated pixels are obtained for all pixel positions becomes a complex process, making a hardware solution difficult.
Another interpolation method detects motion by comparing pairs of reference pixels in the preceding and following frames that are point-symmetric with respect to the interpolation position and takes the mean value of the most highly correlated pair of pixels (see, for example, Japanese Patent Application Publication No. 2006-129181, p. 8, FIG. 3). Because correlation is based on individual pixels, however, high correlations between pixels in dissimilar parts of the image may arise by chance, producing false motion detection and an incorrect interpolated image.
There is a need for a frame interpolation method that can eliminate blur and judder and produce an accurately interpolated image with simple read and write control.